Claims

1. A method for providing thermal protection and monitoring to a three-phase electric motor during startup of the motor, comprising:sampling an electrical signal to the motor;calculating positive-sequence voltage values from the sampled electrical signal during a startup period;calculating positive-sequence current values from the sampled electrical signal during the startup period;calculating resistance values from the calculated positive-sequence voltage values and the calculated positive-sequence current values during the startup period;determining an initial resistance value equal to a minimum of the resistance values calculated during a predetermined initiation period;calculating slip values from the initial resistance value;calculating rotor resistance values from the slip values; and,calculating rotor temperature from the rotor resistance values and current values sampled from the power signal to the motor.

2. The method of claim 1, wherein the step of calculating the resistance values comprises calculating motor resistance values.

3. The method of claim 2, wherein the step of determining an initial resistance value comprises determining an initial motor resistance value equal to a minimum of the motor resistance values calculated during the predetermined initiation period.

4. The method of claim 2, further comprising the step of calculating stator resistance values from the motor resistance values.

5. The method of claim 4, wherein the step of determining an initial resistance value comprises determining an initial stator resistance value equal to a minimum of the stator resistance values calculated during the predetermined initiation period.

6. The method of claim 1, wherein the startup period comprises the predetermined initiation period.

7. The method of claim 6, wherein the predetermined initiation period comprises a period including a number of power system cycles.

8. The method of claim 6, wherein the predetermined initiation period comprises a period including the first ten power system cycles during the startup period.

9. The method of claim 6, wherein the predetermined initiation period comprises a period including the first four power system cycles during the startup period.

10. The method of claim 6, wherein the predetermined initiation period begins after the first power system cycle.

11. The method of claim 10, wherein the predetermined initiation period begins after a predetermined condition is satisfied.

12. The method of claim 1, wherein the step of calculating rotor temperature comprises:calculating positive-sequence rotor resistance values from the slip values;calculating negative-sequence rotor resistance values from the slip values; and,calculating rotor temperature values from the calculated positive-sequence rotor resistance values, the negative-sequence rotor resistance values, and the current values sampled from the power signal to the motor.

13. A system for monitoring a rotor temperature of a rotor of a three-phase electric motor during a startup period, comprising;a three-phase electric motor comprising a rotor, a stator, and an electric power input for providing electric power to the electric motor;a current transformer in communication with the electric power input for providing a current signal;a potential transformer in communication with the electric power input for providing a voltage signal;a first sampler in communication with the current transformer for sampling the current signal to provide current samples;a second sampler in communication with the potential transformer for sampling the voltage signal and to provide voltage samples;a positive-sequence current calculator in communication with the first sampler for calculating positive-sequence current values from the current samples;a positive-sequence voltage calculator in communication with the second sampler for calculating positive-sequence voltage values from the voltage samples;a resistance calculator in communication with the positive-sequence current calculator and the positive-sequence voltage calculator for calculating resistance values from the positive-sequence current values and the positive-sequence voltage values;an initial resistance calculator in communication with the resistance calculator for determining an initial resistance value equal to a minimum of the resistance values during a predetermined initiation period;a slip calculator in communication with the initial resistance calculator and the resistance calculator for determining slip values from the initial resistance value and the resistance values;a rotor resistance calculator in communication with the slip calculator for calculating slip-dependent rotor resistance values from the slip values; anda rotor temperature calculator in communication with the rotor resistance calculator and the first sampler, for calculating rotor temperature values from the current values and the slip-dependent rotor resistance values.

14. The system of claim 13, wherein the resistance calculator comprises a motor resistance calculator and the resistance values comprises motor resistance values.

15. The system of claim 14, wherein the initial resistance value comprises an initial motor resistance value equal to a minimum of the motor resistance values calculated during the predetermined initiation period.

16. The system of claim 14, further comprising a stator resistance calculator in communication with the motor resistance calculator for calculating stator resistance values from the motor resistance values.

17. The system of claim 16, wherein the initial resistance value comprises an initial stator resistance value equal to a minimum of the stator resistance values calculated during the predetermined initiation period.

18. The system of claim 13, wherein the startup period comprises the predetermined initiation period.

19. The system of claim 18, wherein the predetermined initiation period comprises a period including a number of power system cycles.

20. The system of claim 18, wherein the predetermined initiation period comprises a period including the first ten power system cycles during the startup period.

21. The system of claim 18, wherein the predetermined initiation period comprises a period including the first four power system cycles during the startup period.

22. The system of claim 18, wherein the predetermined initiation period comprises a period beginning after the first power system cycle.

23. The system of claim 22, wherein the predetermined initiation period begins after a predetermined condition is satisfied.

24. The system of claim 13, wherein:the rotor resistance calculator is further configured to:calculate positive-sequence rotor resistance values from the slip values; and,calculate negative-sequence rotor resistance values from the slip values; and,the rotor temperature calculator is further configured to calculate rotor temperature values from the calculated positive-sequence rotor resistance values, the negative-sequence rotor resistance values, and the current values sampled from the power signal to the motor.

25. An apparatus for monitoring a rotor temperature of a rotor of a three-phase electric motor during a startup period, comprising;a first sampler in communication with an electric power input to the electric motor for sampling a current signal to provide current samples;a second sampler in communication with the electric power input for sampling a voltage signal and to provide voltage samples;a positive-sequence current calculator in communication with the first sampler for calculating positive-sequence current values from the current samples;a positive-sequence voltage calculator in communication with the second sampler for calculating positive-sequence voltage values from the voltage samples;a resistance calculator in communication with the positive-sequence current calculator and the positive-sequence voltage calculator for calculating resistance values from the positive-sequence current values and the positive-sequence voltage values;an initial resistance calculator in communication with the resistance calculator for determining an initial resistance value equal to a minimum of the resistance values during a predetermined initiation period;a slip calculator in communication with the resistance calculator and the initial resistance calculator for determining slip values from the resistance values and the initial resistance value;a rotor resistance calculator in communication with the slip calculator for calculating slip-dependent rotor resistance values from the slip values; anda rotor temperature calculator in communication with the rotor resistance calculator and the first sampler, for calculating rotor temperature values from the current values and the slip-dependent rotor resistance values.

26. The apparatus of claim 25, wherein the resistance calculator comprises a motor resistance calculator and the resistance values comprises motor resistance values.

27. The apparatus of claim 26, wherein the initial resistance value comprises an initial motor resistance value equal to a minimum of the motor resistance values calculated during the predetermined initiation period.

28. The apparatus of claim 26, further comprising a stator resistance calculator in communication with the motor resistance calculator for calculating stator resistance values from the motor resistance values.

29. The apparatus of claim 28, wherein the initial resistance value comprises an initial stator resistance value equal to a minimum of the stator resistance values calculated during the predetermined initiation period.

30. The apparatus of claim 26, wherein the startup period comprises the predetermined initiation period.

31. The apparatus of claim 30, wherein the predetermined initiation period comprises a period including a number of power system cycles.

32. The apparatus of claim 30, wherein the predetermined initiation period comprises a period including the first ten power system cycles during the startup period.

33. The apparatus of claim 30, wherein the predetermined initiation period comprises a period including the first four power system cycles during the startup period.

34. The apparatus of claim 30, wherein the predetermined initiation period comprises a period beginning after the first power system cycle.

35. The apparatus of claim 34, wherein the predetermined initiation period begins after a predetermined condition is satisfied.

36. The apparatus of claim 25, wherein:the rotor resistance calculator is further configured to:calculate positive-sequence rotor resistance values from the slip values; and,calculate negative-sequence rotor resistance values from the slip values; and,the rotor temperature calculator is further configured to calculate rotor temperature values from the calculated positive-sequence rotor resistance values, the negative-sequence rotor resistance values, and the current values sampled from the power signal to the motor.